make size_and_align_of_mplace work on all projectable

compiler/rustc_const_eval/src/interpret/projection.rs

@@ -168,7 +168,7 @@ where
             // Re-use parent metadata to determine dynamic field layout.
             // With custom DSTS, this *will* execute user-defined code, but the same
             // happens at run-time so that's okay.
-            match self.size_and_align_of(&base_meta, &field_layout)? {
+            match self.size_and_align_from_meta(&base_meta, &field_layout)? {
                 Some((_, align)) => {
                     // For packed types, we need to cap alignment.
                     let align = if let ty::Adt(def, _) = base.layout().ty.kind()